Showing papers in "Reviews of Geophysics in 2006"

Modeling non‐Fickian transport in geological formations as a continuous time random walk

Abstract: [1] Non-Fickian (or anomalous) transport of contaminants has been observed at field and laboratory scales in a wide variety of porous and fractured geological formations. Over many years a basic challenge to the hydrology community has been to develop a theoretical framework that quantitatively accounts for this widespread phenomenon. Recently, continuous time random walk (CTRW) formulations have been demonstrated to provide general and effective means to quantify non-Fickian transport. We introduce and develop the CTRW framework from its conceptual picture of transport through its mathematical development to applications relevant to laboratoryand field-scale systems. The CTRW approach contrasts with ones used extensively on the basis of the advectiondispersion equation and use of upscaling, volume averaging, and homogenization. We examine the underlying assumptions, scope, and differences of these approaches, as well as stochastic formulations, relative to CTRW. We argue why these methods have not been successful in fitting actual measurements. The CTRW has now been developed within the framework of partial differential equations and has been generalized to apply to nonstationary domains and interactions with immobile states (matrix effects). We survey models based on multirate mass transfer (mobile-immobile) and fractional derivatives and show their connection as subsets within the CTRW framework.

Storms in the ionosphere: Patterns and processes for total electron content

Abstract: [1] The ionosphere's total electron content (TEC) is a parameter widely used in studies of the near-Earth plasma environment. The scientific use of TEC appeared early in the artificial satellite era, and among its many contributions were fundamental insights into how the ionosphere responds to geomagnetic storms. While many excellent reviews of solar-terrestrial disturbances exist in the literature, none have concentrated on the TEC parameter per se. With new TEC data sets increasingly available from the Global Positioning System (GPS), a comprehensive summary of pre-GPS storm studies is needed to set the base for progress in the GPS era. This review summarizes past case studies, describes statistical occurrence pattern, and identifies responsible mechanisms validated via modeling. It presents a new set of results of TEC disturbance patterns during 180 geomagnetic storms to describe seasonal and solar cycle effects. It concludes with a set of open questions that require additional study.

Representative hydraulic conductivities in saturated groundwater flow

Abstract: [1] Heterogeneity is the single most salient feature of hydrogeology. An enormous amount of work has been devoted during the last 30 years to addressing this issue. Our objective is to synthesize and to offer a critical appraisal of results related to the problem of finding representative hydraulic conductivities. By representative hydraulic conductivity we mean a parameter controlling the average behavior of groundwater flow within an aquifer at a given scale. Three related concepts are defined: effective hydraulic conductivity, which relates the ensemble averages of flux and head gradient; equivalent conductivity, which relates the spatial averages of flux and head gradient within a given volume of an aquifer; and interpreted conductivity, which is the one derived from interpretation of field data. Most theoretical results are related to effective conductivity, and their application to real world scenarios relies on ergodic assumptions. Fortunately, a number of results are available suggesting that conventional hydraulic test interpretations yield (interpreted) hydraulic conductivity values that can be closely linked to equivalent and/or effective hydraulic conductivities. Complex spatial distributions of geologic hydrofacies and flow conditions have a strong impact upon the existence and the actual values of representative parameters. Therefore it is not surprising that a large body of literature provides particular solutions for simplified boundary conditions and geological settings, which are, nevertheless, useful for many practical applications. Still, frequent observations of scale effects imply that efforts should be directed at characterizing well-connected stochastic random fields and at evaluating the corresponding representative hydraulic conductivities.

Dust devils on Earth and Mars

Abstract: [1] Dust devils, particle-loaded vertical convective vortices found on both Earth and Mars, are characterized by high rotating wind speeds, significant electrostatic fields, and reduced pressure and enhanced temperature at their centers. On Earth they are subordinate to boundary layer winds in the dust cycle and, except possibly in arid regions, are only “nuisance-level” phenomena. On Mars, though, they seem to support the persistent background atmospheric haze, to influence the surface albedo through the formation of “tracks” on the surface, and to possibly endanger future exploration because of their high dust load and large potential gradients. High-resolution numerical simulations and thermophysical scaling models successfully describe dust devil–like vortices on Mars, but fitting dust devil action into the Martian global dust cycle is still problematic. Reliable parameterizations of their erosional abilities and solid temporal and spatial distribution data are still required to build and test a complete model of dust devil action.

Postperovskite phase transition and its geophysical implications

Abstract: [1] The stability of (Mg,Fe)SiO3 perovskite in the deep lower mantle has long been uncertain. Recently, a phase transition from perovskite to postperovskite was discovered through a significant change in the X-ray diffraction pattern at high-pressure and high-temperature conditions corresponding to the core-mantle boundary region. This phase transition was also confirmed by first-principles calculations. These suggest that (Mg,Fe)SiO3 postperovskite is the predominant mineral in the lowermost mantle called the D″ layer, and phase transition from perovskite to postperovskite has significant geophysical implications for its nature and dynamics. The postperovskite phase may account for the large seismic anomalies observed in the D″ region, such as the D″ discontinuity, S wave anisotropy, and anticorrelation between the anomalies in S wave and bulk sound velocities. In addition, this phase transition is a strongly exothermic reaction. It destabilizes the thermal boundary layer at the base of the mantle and should promote the formation of high-temperature upwelling plumes.

Subduction evolution and mantle dynamics at a continental margin: Central North Island, New Zealand

Abstract: [1] Central North Island, New Zealand, provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin Whereas most subduction zones are innately two-dimensional, North Island of New Zealand displays a distinct three-dimensional character in the back-arc regions Specifically, we observe “Mariana-type” subduction in the back-arc areas of central North Island in the sense of back-arc extension, high heat flow, prolific volcanism, geothermal activity, and active doming and exhumation of the solid surface Evidence for emplacement of a significant percent of new lithosphere beneath the central North Island comes from heat flux of 25 MW/km of strike (of volcanic zone) and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust Seismic attenuation (Qp−1) is high (∼240), and rhyolitic and andesitic volcanism are widespread Almost complete removal of mantle lithosphere is inferred here in Pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 74 ± 01 km/s In contrast, southwestern North Island exhibits “Chilean-type” back-arc activity in the sense of compressive tectonics, reverse faulting, low-heat-flow, thickened lithosphere, and strong coupling between the subducted and overriding plates This rapid switch from Mariana-type to Chilean-type subduction occurs despite the age of the subducted plate being constant under North Island Moreover, stratigraphic evidence shows that processes that define the extensional back-arc area (the Central Volcanic Region) are advancing southward into the compressional system (Wanganui Basin) at about 10 mm/yr We link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back-arc regions Thickening occurred during the Miocene within the Taranaki Fault Zone The process of thickening and convective removal is time- and temperature-dependent and has left an imprint in both the geological record and geophysical properties of central North Island, which we document and describe

The paleomagnetism of single silicate crystals: recording geomagnetic field strength during mixed polarity intervals, superchrons, and inner core growth

Abstract: [1] The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Commonly accepted results have come from analyses of bulk samples of lava. Historic lavas have been shown to faithfully record the past field strength when analyzed using the Thellier double-heating method. Data from older lavas, however, tend to show effects of in situ and laboratory-induced alteration. Here we review an alternative approach. Single plagioclase crystals can contain minute magnetic inclusions, 50–350 nm in size, that are potential high-fidelity field recorders. Thellier experiments using plagioclase feldspars from an historic lava on Hawaii provide a benchmark for the method. Rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples. This resistance is likely related to the lack of clays. In addition, magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well-defined thermoremanent magnetization into an irresolute chemical remanent magnetization. If there is a relationship between geomagnetic reversal frequency and paleointensity, it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (∼83–120 million years ago) suggest a strong (>12 × 1022 Am2), stable field, consistent with an inverse relationship between reversal frequency and paleointensity. Superchrons may represent times when the pattern of core-mantle boundary heat flux allows the geodynamo to operate at peak efficiency, as suggested in some numerical models. Thellier data from single plagioclase crystals formed during times of moderate ( 10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These paleointensity data, together with a consideration of paleomagnetic directions, suggest that geomagnetic reversals, field morphology, secular variation, and intensity are related. The linkages over tens of millions of years imply a lower mantle control on the geodynamo. On even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the Proterozoic and Archean geomagnetic field during the onset of growth of the solid inner core. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field similar to that of the modern, 2.5–2.7 billion years ago. Older Archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow. Most rocks of this age have been affected by low-grade metamorphism; investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo. Absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization, anisotropy, and slow cooling. Fortunately, prior work in rock magnetism, together with advances in analytical equipment and techniques, provides a solid foundation from which these frontier issues can be approached.

Optical oceanography: Recent advances and future directions using global remote sensing and in situ observations

Abstract: [1] The present review describes progress in addressing and solving several fundamental and applied problems involving optical oceanography. These problems include: primary productivity, ecosystem dynamics, biogeochemical cycling, upper ocean heating, and the impacts of anthropogenic disturbances on ocean dynamics. Technological advances in optical sensors and ocean observing platforms are being used to increase the variety and quantity of optical observations and to greatly expand their sampling capabilities in time and space. Remote sensing of ocean color from aircraft- and satellite-borne instruments is vital to obtain regional- and global-scale optical data synoptically. In situ observations provide complementary subsurface data sets with high temporal and spatial resolution. In situ observations are also essential for calibration and validation of remotely sensed data as well as for algorithm development and data assimilation models. Important challenges remain to synthesize regional and global optical data sets obtained from optical sensors and oceanographic platforms and to utilize these data sets in predictive models of oceanic optical, physical, and biogeochemical dynamics.

Systems theory for geospace plasma dynamics

Abstract: [1] This is a tutorial review on systems theory and its applications to space plasma physics and, more broadly, on geophysics. With its basis on the state representation of a plasma the theory is widely applicable, but it is of particular interest for dynamical, nonlinear, or out-of-equilibrium regimes that cannot be represented by traditional microscopic modeling. Two distinct, but closely related, branches of the theory are applied when the plasma dynamics is traced to first principles and when it needs to be derived from experimental data. A framework of modeling methods is presented in order of increasing complexity: enumeration of the effective degrees of freedom, measurement of the linear dynamics and stability, and generalization to their nonlinear counterparts. The relation between symmetries in the plasma system and modes in its structure and response is discussed. Signal processing methods are presented, illustrated by examples, and their relative merits and limitations are discussed. The dynamical framework provides a new approach alongside the traditional perturbative and statistical-mechanical methodologies and is directly relevant to the development of space weather applications.

Formation of refrozen snowpack layers and their role in slab avalanche release

Abstract: [1] In a variety of snow climates, numerous slab avalanches release over crusts consisting of refrozen snow. Slab avalanches sometimes release in weak layers of faceted crystals that developed while underlying wet layers froze into crusts, often within a day. Weak layers of faceted crystals can also develop when less permeable and more conductive crusts alter the temperature and vapor pressure gradients. These processes create interfaces where differences in grain radii can contribute to weak bonding. In western Canada, layers of faceted crystals (facet layers) on crusts are most common in early and late winter when thaws and rain are more frequent. Also, thin facet layers occur in spring when surface melting by solar radiation is common. Shear strength tests on facet layers show an initial strength loss during faceting followed by a slow strength increase. The spatial distribution of poorly bonded crusts can be interpreted from the interaction of terrain and meteorology that caused the antecedent wet layer.